1. Field of the Invention
The present invention relates generally to radio communications techniques, and more particularly, to a method and an apparatus for transmitting Hybrid Automatic Repeat reQuest (HARQ) indication information.
2. Description of the Related Art
In 3rd Generation Partnership Project (3GPP) Long-Term Evolution (LTE)/LTE-Advanced systems, each radio frame has a length of 10 ms which is equally divided into 10 subframes. One downlink Transmission Time Interval (TTI) is defined for each subframe. FIG. 1 is a diagram illustrating a frame structure of a Frequency Division Duplexing (FDD) system. Each downlink subframe is formed of two slots. For a normal Cyclic Prefix (CP), each slot includes 7 Orthogonal Frequency Division Multiplexing (OFDM) symbols. For an extended CP, each slot includes 6 OFDM symbols. FIG. 2 shows a frame structure of a Time Division Duplexing (TDD) system. Each radio frame is equally divided into two half frames with a length of 5 ms. Each of subframe 1 and subframe 6 respectively includes 3 special fields, i.e., a Downlink Pilot Time Slot (DwPTS), a Guard Period (GP), and an Uplink Pilot Time Slot (UpPTS). The total length of the three special fields is 1 ms.
FIG. 3 is a diagram illustrating a structure of a downlink subframe of an LTE system. OFDM symbols (n=1, 2, or 3) at the forefront of the downlink subframe belong to a downlink control channel area and are used for transmitting user downlink control information including a Physical Downlink Control CHannel (PDCCH), a Physical Control Format Indicating CHannel (PCFICH) and Physical HARQ Indicating CHannel (PHICH). Remaining OFDM symbols are used for transmitting user downlink data, e.g., a Physical Downlink Shared CHannel (PDSCH). Downlink physical channels are a set of Resource Elements (REs). An RE is a minimum unit of the time-frequency resources, i.e., a subcarrier in frequency domain and an OFDM symbol in time domain. The unit for physical resource allocation is a Physical Resource Block (PRB). One PRB includes 12 consecutive subcarriers in the frequency domain and corresponds to one slot in the time domain. Two PRBs on the same subcarriers of two slots within one subframe are referred to a PRB pair. The REs may be used for different purposes. Some REs do not correspond to the above mentioned physical channels but are merely used for transmitting downlink physical signals including a downlink reference signal and a synchronizing signal. According to different functions, the LTE downlink reference signal includes a cell-specific Common Reference Signal (CRS), a user-specific Demodulation Reference Signal (DMRS) and a Channel State Indication Reference Signal (CSI-RS).
In the LTE system, uplink data is transmitted based on a synchronous HARQ scheme. The initial transmission is triggered by the PDCCH carrying an UpLink (UL) grant. Retransmission may be triggered by the UL grant and the PHICH. In the LTE system, the semi-statically reserved PHICH time-frequency resources are indicated in a Physical Broadcast Channel (PBCH), i.e., 2 bits are used for indicating the number of PHICH groups. The RE resources of each PHICH group consist of 3 Resource Element Groups (REGs), which are distributed on the whole downlink bandwidth with equal intervals, so as to obtain a relatively large frequency diversity gain. Each REG is formed of 4 or 6 adjacent REs in one OFDM symbol. For a REG is formed of 6 adjacent REs, two REs are occupied by the CRS. Therefore, there are only 4 available REs. 8 PHICH resources may be multiplexed on each PHICH group through orthogonal extension. The PHICH resources allocated to one User Equipment (UE) are determined according to a minimum PRB index of the Physical Uplink Shared CHannel (PUSCH) and the uplink reference signal indication information (nDMRS) in the UL grant. In particular, each PHICH resource is identified by an index pair (nPHICHgroup, nPHICHseq), wherein nPHICHgroup denotes the index of the PHICH group, nPHICHseq denotes the index of the orthogonal sequence in the PHICH group. Thus, the PHICH resources occupied by the UE can be defined according to following Equations (1) and (2)nPHICHgroup=(IPRB_RAlowest_index+nDMRS)modNPHICHgroup+IPHICHNPHICHgroup nPHICHseq=(└IPRB_RAlowest_index/NPHICHgroup┘+nDMRS)mod2NSFPHICH   (1) and (2)
Herein, nDMRS denotes the uplink reference signal indication information, nSFPHICH denotes a spread factor of the PHICH, IPRB_RAlowest_index denotes a minimum PRB index in slot 0 of the PUSCH, NPHICHgroup denotes the number of PHICH groups semi-statically configured according to the following Equation (3):
                              I          PHICH                =                  {                                                                      1                                                                                                                                              used                          ⁢                                                                                                          ⁢                          for                          ⁢                                                                                                          ⁢                          transmission                          ⁢                                                                                                          ⁢                          of                          ⁢                                                                                                          ⁢                          PUSCH                          ⁢                                                                                                          ⁢                          in                          ⁢                                                                                                          ⁢                          subframes                          ⁢                                                                                                          ⁢                          4                          ⁢                                                                                                          ⁢                          and                          ⁢                                                                                                          ⁢                          9                                                                                                                                                              of                          ⁢                                                                                                          ⁢                          TDD                          ⁢                                                                                                          ⁢                          uplink                          ⁢                                                      /                                                    ⁢                          downlink                          ⁢                                                                                                          ⁢                          configuration                          ⁢                                                                                                                                            ⁢                                                                                                                                          ⁢                          0                                                                                                                                                                  0                                                  others                                                      ⁢                                                                                    (        3        )            
In order to support a larger capacity of the control channel and support interference coordination of control channels of multiple cells, an enhanced PDCCH is proposed, referred to as ePDCCH hereinafter. The ePDCCH is mapped in the data area of the subframe for transmission and is multiplexed with the PDSCH in a Frequency Division Multiplex (FDM) manner. The base station may inform the UE of the PRB pair used for transmitting the ePDCCH through higher layer signaling. For different UEs, the PRB pair used for transmitting the ePDCCH may be different.
In order to configure the ePDCCH, a concept of an ePDCCH set is proposed. The base station may configure the UE to detect the ePDCCH on two ePDCCH sets. Each ePDCCH set is formed of NPRB PRB pairs, e.g., NPRB=2, 4, or 8. According to the method for mapping the ePDCCH, the ePDCCH may include a local ePDCCH and a distributed ePDCCH. Each ePDCCH set is either used for bearing the distributed ePDCCH or used for bearing the local ePDCCH. Each distributed ePDCCH is generally mapped to all PRB pairs of one ePDCCH set, whereas the local ePDCCH is centrally mapped to one PRB pair of the ePDCCH set. If the aggregation levels of the local ePDCCH are relatively large, the local ePDCCH may also be mapped to multiple PRB pairs of the ePDCCH set.
In order to attain multiple ePDCCHs multiplexed on one PRB pair, except for the REs used for the DMRS, the REs in each PRB pair are divided into RE groups, referred to as enhanced REGs (eREGs). As shown in FIG. 4, each PRB pair is divided into 16 eREGs with indexes from 0 to 15. The indexes of the eREGs are mapped to all REs in the PRB pair except for the REs used for the DMRS firstly according to the frequency and then according to the time. In the case that normal CP is adopted, except for 24 RE resources used for the DMRS, a total of 144 RE resources are remained in the PRB pair. Therefore, each eREG includes 9 RE resources. A Control Channel Element (CCE) is obtained through combining multiple eREGs, denoted by enhanced CCE (eCCE). Through combining multiple eCCEs, the time-frequency resource occupied by one ePDCCH may be obtained. One eCCE is formed of NeREG eREGs. With respect to different frame structures, NeREG=4 or 8. The 16 eREG indexes may be divided into 4 groups. eREG group 0 includes eREG indexes {0, 4, 8, 12}, eREG group 1 includes eREG indexes {1, 5, 9, 13}, eREG group 2 includes eREG indexes {2, 6, 10, 14}, eREG group 3 includes eREG indexes {3, 7, 11, 15}. Thus, if one eCCE is formed of 4 eREGs, the eCCE is formed by one group among the 4 eREG groups. If one eCCE is formed of 8 eREGs, the eCCE is formed by two pairs of eREG groups which are respectively eREG group (0, 2) and eREG group (1, 3). In other words, one eCCE includes eREG indexes {0, 2, 4, 6, 8, 10, 12, 14}, and another eCCE includes eREG indexes {1, 3, 5, 7, 9, 11, 13, 15}.
In evolved systems of the LTE system, the overhead of subsequent compatibility control signaling and the CRS is reduced, and interferences brought by the subsequent compatibility control signaling and the CRS are also reduced, which increases the spectrum utilization ratio of the UE. Since the CRS overhead is reduced, system power saving performance is improved. In such a system, the ePDCCH and the PDSCH are generally demodulated based on the DMRS. At present, it is referred to as a New Carrier Type (NCT).
In the NCT system, an enhanced PHICH (ePHICH) must be configured in a manner corresponding to the ePDCCH. Thus, when uplink data is transmitted in the NCT cell based on the synchronous HARQ, the ePHICH is used for acknowledgement of the correct reception of the uplink data or for triggering non-adaptive retransmission of the uplink data. For example, if the NCT cell works as a Secondary Cell (Scell) in a Carrier Aggregation (CA) system and the uplink transmission of the NCT cell is scheduled according to a self-scheduling policy, according to the existing design, the ePHICH resources must be allocated on the cell transmitting the UL grant, i.e., the current NCT cell. Therefore, there is a need for a way to transmit the ePHICH in the NCT system.